Magnesium alloy sheet and its production

ABSTRACT

A method of producing magnesium alloy strip, suitable for use in the production of magnesium alloy sheet by rolling reduction and heat treatment, involves casting magnesium alloy as strip, using a twin roll casting installation. In the casting, the thickness and temperature of the strip exiting from between rolls of the installation are controlled whereby the strip has a microstructure characterised by a primary phase having a form selected from deformed dendritic, equiaxed dendritic and a mixture of deformed and equiaxed dendritic forms. The resultant strip is amenable to production of sheet material by application of a homogenizing heat treatment followed by rolling and annealing.

This invention relates to magnesium alloy sheet and to a process for its production.

The most common approach to the production of magnesium alloy sheet involves hot rolling of an ingot produced by pouring a melt of the alloy into a suitable mould. The ingot is subjected to a homogenizing soak at a suitable elevated temperature and then scalped to obtain clean, smooth surfaces. The scalped ingot is rolled to produce plate, then strip and finally sheet by a rough hot rolling treatment, followed by hot intermediate/finish rolling, and a final anneal. In some instances, the hot intermediate rolling is followed by cold rolling to enable the reduction to the final gauge of the resultant sheet to be fine tuned.

In that approach, the ingot may for example be up to 1800 mm long, 1000 mm wide and up to 300 mm thick. The homogenization heat treatment usually is from 400° C. to 500° C. for up to 2 hours. The scalping usually is to a depth of about 3 mm. The rough hot rolling, at from about 400° C. to 460° C., is able to achieve a substantial reduction in each pass, such as from 15% to 40%, generally about 20%, in as many as 25 passes, to produce flat plate of about 5 mm thick. When necessary to maintain the temperature above the 400° C. minimum, the alloy is reheated between passes.

The rough hot rolling usually is followed by intermediate hot rolling at 340° C. to 430° C., to reduce flat plate to strip of about 1 mm thick. In each of up to about 10 passes, a reduction of about 8% to 15%, generally about 10% is achieved. Reheating is necessary after each pass in order to maintain the temperature above the 340° C. minimum.

The intermediate hot rolling is followed by finish rolling, to reduce the strip to sheet of a final gauge of about 0.5 mm, by either warm rolling or cold rolling. The finish warm rolling is conducted at from 190° C. to 400° C. In this, the strip is reduced in each of from 10 to 20 passes by from 4% to 10%, usually about 7%. Again, heating between each pass is necessary due to rapid cooling of the thin alloy. Care in reheating is necessary as overheating can result in excessive reduction and loss of control over the gauge. Cold rolling can be preferred to enable fine tuning to the final gauge, but this necessitates only 1% to 2% thickness reduction in each pass and, hence, a larger number of passes to the final gauge.

The rough hot rolling stage is quite efficient, despite the high number of passes, since there is only limited cooling between passes and the lower rate of heat loss necessitates reheating after only a small proportion of the passes. However, the intermediate hot rolling necessitates a substantial consumption of energy as a coil mill is employed in processing the 5 mm plate down to 1 mm strip, and heat losses necessitate heating before each pass which significantly prolongs the overall process of producing sheet. Also, the intermediate hot rolling can result in surface and edge cracking of the strip, and a resultant reduction in metal yield. These problems in the intermediate hot rolling are exacerbated in the finish warm rolling and, while this is not the case in finish cold rolling, there is the added cost of a larger number of passes necessary in the cold rolling.

The final anneal, after the finish warm or cold rolling varies according to the intended application for the magnesium alloy sheet produced. The final anneal may be an O temper requiring heating at about 370° C. for one hour; an H24 temper by heating at about 260° C. for one hour; or an H26 temper by heating at about 150° C. for one hour. However, there is ample scope for variation of the final anneal to achieve resultant sheet having the mechanical properties desired for different applications.

The time and energy consumption for the production of magnesium alloy sheet by the above production stages is relatively large. As a consequence, the cost of production of the sheet is high relative to that for aluminium sheet, for example. The present invention seeks to provide a process for the production of magnesium alloy sheet which reduces the level of consumption of time and energy and thereby enables more cost effective production of the sheet.

There have been proposals for the production of magnesium alloy plate and strip by twin roll casting (TRC). The TRC process does not enable the direct production of magnesium alloy sheet, since the benefits of TRC do not favour producing strip thinner than about 1 to 2 mm. Despite this, TRC suggests a possible alternative to the above described process which has the benefit of eliminating the stages of ingot production, homogenizing heat treatment, scalping and the rough hot rolling stage by utilising TRC strip as the feed for subsequent processing to sheet. That is, in terms of gauge, the output from TRC ranges from being comparable to the plate obtained after that rough hot rolling stage down to strip resulting from the intermediate warm rolling stage. However, the TRC strip differs significantly from either of the plate resulting from the rough hot rolling, or the strip resulting from the intermediate warm rolling, of ingot alloy and is too variable in its microstructure to enable simple reliance on that alternative.

The as-cast TRC strip is found to vary in its microstructure with its casting conditions. In addition to this overall variability, it is not completely uniform throughout its thickness. It contains dendrites of different sizes and discontinuous or a varying amount of segregation from the surfaces towards the centre. Also, the as-cast TRC strip is prone to the generation of surface cracks during rolling with even a small reduction, and any segregation adversely affects the ductility of the finished strip. Thus, a homogenization heat treatment is necessary prior to any rolling schedule, although this is found not to fully offset the variation in microstructure and the resultant difficulty in rolling.

We have found that TRC magnesium strip, with a suitable microstructure which enables the production of sheet, can be obtained by control over the conditions under which the strip is produced. A suitable microstructure is found to be related to the secondary dendritic arm spacing and the amount of rolling reduction achieved in producing the as-cast strip, with the suitable microstructure reflected by the temperature at which the strip exits from the rolls. We also have found that with attainment of a suitable microstructure, the as-cast TRC strip after a homogenization heat treatment is substantially more amenable to being rolled and annealed to produce suitable magnesium alloy sheet.

Thus according to the present invention, there is provided a method of producing magnesium alloy strip, suitable for use in the production of magnesium alloy sheet by rolling reduction and heat treatment, wherein the method includes the steps of:

(a) casting magnesium alloy as strip, using a twin roll casting installation; and

(b) controlling the thickness and temperature of the strip exiting from between rolls of the installation whereby the strip has a microstructure characterised by a primary phase having a form selected from deformed dendritic, equiaxed dendritic and a mixture of deformed and equiaxed dendritic forms.

A suitable microstructure having “deformed” and/or “equiaxed” dendritic primary phase is able to be produced with a roll exit temperature of from about 200° C. to 350° C., such as from about 200° C. to 260° C. A deformed dendritic microstructure, substantially free of equiaxed dendritic particles, is obtained with a relatively low exit temperature which varies with the thickness of the strip. For thicker strip, such as about 4 mm to 5 mm thick, the deformed dendritic microstructure tends to be obtained at a temperature of from about 200° C. to 220° C. For thinner strip, the deformed dendritic microstructure tends to be obtained at from about 200° C. to 245° C., more usually above about 220° C. An equiaxed microstructure, substantially free of deformed dendritic particles, generally is obtained with a relatively high exit temperature which also varies with the strip thickness. For thicker strip such as about 4 mm to 5 mm thick, the equiaxed dendritic microstructure tends to be obtained at a temperature of at least about 230° C. and, for this microstructure and thickness, it is preferred that the exit temperature is at an intermediate level of from about 230° C. to 240° C. At higher exit temperatures for such thicker strip, particularly at a high level of from about 250° C. to 260° C., there is increased segregation in grain boundaries near to the surfaces of the as-cast strip. For thinner strip, the equiaxed dendritic microstructure tends to be obtained at exit temperatures higher than about 245° C., and with a lesser tendency for segregation in grain boundaries near to surfaces of the as-cast strip.

The equiaxed dendritic microstructure has primary phase grains which, rather than exhibiting a shape reflecting dendritic crystal growth, are somewhat rounded and of substantially uniform size in all directions. The deformed dendritic microstructure has primary phase grains which have a shape which more clearly reflects dendritic crystal growth. However, the deformed primary grains are of an elongate flattened form extending in the rolling direction, substantially parallel to major surfaces of the strip.

The deformed dendritic microstructure is preferred. It is amenable to the production of magnesium alloy sheet by a more simple form of the invention. Also, the equiaxed dendritic microstructure is more prone to micro-cracking near the surfaces of the as-cast strip, particularly at exit temperatures of 240° C. to 250° C., with the micro-cracking appearing in the segregation regions in grain boundaries.

In the present invention, magnesium alloy TRC strip is produced to a suitable thickness of less than 10 mm, under conditions providing a suitable microstructure. The strip then is subjected to a homogenization heat treatment to achieve full or partial recrystallization to an appropriate grain size. The homogenized strip then is rolled to produce magnesium alloy sheet of a required gauge, and the sheet is subjected to a final anneal.

Thus, the present invention also provides a method of producing magnesium alloy sheet, wherein the method includes the steps of:

-   (a) casting magnesium alloy as strip, using a twin roll casting     installation; -   (b) controlling the thickness and temperature of the strip exiting     from between rolls of the installation whereby the strip has a     microstructure characterised by a primary phase having a form     selected from deformed dendritic, equiaxed dendritic and a mixture     of deformed and equiaxed dendritic forms; -   (c) subjecting the strip to a homogenizing heat treatment to achieve     full or partial recrystallization of the microstructure to a     required grain size; -   (d) rolling the homogenized strip to produce magnesium alloy sheet     of a required gauge; and -   (e) annealing the sheet produced by step (d).

The as-cast magnesium alloy strip preferably has a thickness of not more than 5 mm. The thickness most preferably is less than 5 mm, such as down to about 2.5 mm. The microstructure is one characterised by deformed dendritic and/or equiaxed dendritic primary phase. The primary phase may substantially comprise equiaxed dendritic primary phase produced by strip of 4 mm to 5 mm thickness exiting the twin rolls having a temperature of from 230° C. to 260° C., preferably from 230° C. to 240° C. However, the primary phase preferably substantially comprises deformed dendritic primary phase produced by the strip exiting the rolls at a temperature of from 200° C. to 245° C. for thin strip less than 3 mm thickness and from 200° C. to 220° C. for strip thicknesses between 4 mm and 5 mm.

The homogenization heat treatment preferably is at a temperature of from about 330° C. to 500° C., preferably from about 400° C. to 500° C. The strip preferably is subjected to the heat treatment sufficiently soon after exiting the twin rolls so as to minimise loss of heat energy from the as-cast strip, to thereby minimise the time and heat energy input required to obtain the homogenization temperature. However, even if a relatively high temperature of 400° C. to 500° C. is desirable, it can be beneficial for the strip to be held for a period at an intermediate temperature, such as at about 340° C. to 360° C., before heating to the higher temperature, as the intermediate temperature hold enables the level of segregation in some alloys, such as AZ series alloys, to be reduced by secondary phase being taken into solid solution.

The period of time required for the homogenization heat treatment decreases with increasingly higher heat treatment temperature, but differs with the microstructure. With, for example, the deformed dendritic microstructure, the heat treatment results in recrystallization. At a temperature of about 420° C., the recrystallization can be well advanced over a period of only about 2 hours, and tends to be preferentially in regions associated with finer cells. A few large, isolated equiaxed dendrites within the deformed dendrites become individual solid grains, although remnants of the dendritic structure are still visible within the grains. After 6 hours at 420° C., the large grains begin to recrystallize. After 16 hours at 420° C., the final microstructure obtained by heat treatment of the deformed dendritic microstructure is more uniform and consists of fine grains of about 10 μm to 15 μm in size. In addition to this microstructural transformation, it is found that the segregation in some alloys, such as the AZ series alloys, is able to be almost eliminated after the annealing for 2 hours at 420° C., except for a few particles.

The relatively rapid elimination of segregation in the heat treatment of the TRC magnesium alloy strip is in marked contrast to experience with TRC aluminium alloys in which segregation is very significant and not able to be removed by homogenization heat treatment. This is found to result from secondary particles precipitating in an early stage of solidification in the production of TRC magnesium alloys, such that those particles are relatively uniformly distributed over the entire strip cross-section. In contrast, secondary particles are formed in a later stage in the solidification of aluminium alloys and are relatively concentrated in the centre of the thickness of as-cast TRC aluminium alloy strip.

The microstructural transformation during the homogenizing heat treatment is different with TRC magnesium alloy having the equiaxed dendrite microstructure. In contrast to the microstructure having the deformed dendritic structure, the larger grains of the equiaxed microstructure do not recrystallize into smaller grains. Rather, the homogenizing heat treatment results in a final microstructure containing mainly large grains of about 50 μm to 200 μm in size.

After the homogenizing heat treatment, the TRC strip can be subjected to further rolling finishing which is the same for each microstructure type. Where this is the case, the further processing includes stages of finish hot rolling, finish cold rolling and a final anneal. However, the finish hot rolling can be omitted for both the deformed and equiaxed dendritic microstructures. The finish cold rolling of the deformed microstructure can be further improved by using a larger rolling reduction between the interval anneals than for the equiaxed microstructures, to provide a most cost-effective form of the invention. Also, in the case of the equiaxed dendritic microstructure it can be beneficial, in at least some circumstances, to scalp the strip to remove a surface layer, before the finish hot rolling.

The finish hot rolling may be conducted at a temperature at which the rolling causes continuous recrystallization, such that dislocations remain within the recrystallized grains. Generally this necessitates hot rolling temperatures above 200° C. However, the hot rolling usually is at a temperature of from about 350° C. to 500° C., preferably from about 400° C. to 500° C.

With the equiaxed dendritic grain structure, it is necessary to distinguish between TRC strip produced with a roll exit temperature in the lower and upper parts, respectively, of the temperature range of 230° C. to 260° C. For at least some magnesium alloys, strip produced with a lower roll exit temperature, of from about 230° C. to 240° C. for example, is found not to be able to undergo finish hot rolling, even after an extended homogenization heat treatment, unless the strip first is scalped to remove a sufficient surface layer, such as to a depth of about 3 mm. However, again for at least some alloys, scalping is found not to be necessary for strip produced with a higher roll exit temperature, such as from about 250° C. to 260° C.

The need for scalping of strip which, as cast, had an equiaxed dendritic microstructure produced at a lower roll exit temperature such as from 230° C. to 240° C., arises from the surface defects in the strip which are not cured by the homogenization heat treatment. Both large (40%) and small (5%) reductions per pass in the hot rolling are found to produce cracks in the surface of the strip. We have observed that cracks appear after just one pass at the large reduction setting and after only two passes at the small reduction setting. However, suggestive of surface defects, it is found that the detrimental effects of the surface cracks can be minimised by scalping, as indicated above. Moreover, strip cast with a higher exit temperature, such as from about 250° C. to 260° C., is found after homogenization heat treatment to be able to be successfully subjected to a hot rolling reduction of up to 25% per pass without displaying surface cracks.

The finish hot rolling, particularly where conducted at a relatively high temperature, is able to achieve a relatively high actual reduction per pass, such as from 20% to 25%. To illustrate this, test samples of AZ31B strip 330 mm long, 120 mm wide and 4.7 mm thick (after scalping when necessary), were prepared from TRC strip which, as-cast, had an equiaxed dendritic microstructure and which was subjected to a homogenization heat treatment at about 420° C. Each sample was hot rolled at 420° C. to produce sheeting to a total length of about 2000 mm, a width of 120 mm and a thickness of from 0.7 to 0.75 mm. A mill speed of 18 m/min was determined to be sufficient for the hot rolling, at the initial temperature of 420° C. In the first pass, the reduction setting for the mill was between 40% and 45% of the strip thickness, and this was increased to 50% for the second pass and to 60% for the third pass. The actual reduction achieved in the strip for each pass was between 20% and 25%. An intermediate anneal at 420° C. for 30 minutes was conducted between passes one and two, and two and three. In the subsequent three passes, the reduction setting was further increased to between 70% and 90% until the mill gauge was between 0.13 mm and 0.15 mm (0.005″ to 0.006″), with the work piece being re-heated to 420° C. after each pass. The actual reduction in the subsequent three passes was in the order of 17%, which is less than the previous three passes, but it was considered that thinner sheet would lose heat more quickly, resulting in less rolling reduction. In a final four rolling passes, the mill gauge was maintained at between 0.13 mm and 0.15 mm until the sheet thickness reached between 0.7 mm and 0.75 mm. The actual amount of reduction per pass decreased from 15% to 8% as the sheet became thinner.

Further trials were conducted with samples from TRC AZ31B alloy, but produced from TRC strip having a deformed, rather than equiaxed, dendritic as-cast microstructure. Some of the test samples were 200 mm long, 50 mm wide and 2.6 mm thick, while other larger samples were as detailed in the above trials with equiaxed microstructures. With each sample size, two sets of samples were subjected to an homogenizing heat treatment by an overnight anneal, one set at 350° C. and the other at 420° C. The samples then were subjected to the same hot rolling schedule (with respect to the reduction settings for the mill) as described previously, but at two temperature levels of 350° C. and 420° C., to reach a sheet thickness of between 0.7 mm and 0.75 mm. For the smaller samples a reduction of between 21% and 26% was measured per pass for each of the first four passes, followed by one more pass of between 17% and 19% reduction.

The pre-rolling annealing temperature was found to influence the formation of a “banded” microstructure. The “banded” microstructure in the samples annealed at 350° C. before rolling was obvious and persisted even after further cold rolling processing. In the sample annealed at 420° C., the large grains were more uniformly distributed. Hot rolling at an initial temperature of 350° C. also introduced the “banded” microstructure.

Decreasing the duration of the pre-rolling anneal from about 18 hours to 2 hours was found to not affect the rolling reduction and the surface quality. The microstructures, however, contained significant amount of bands of large grains.

Reducing the interval anneal time from 15 to 30 minutes to 7 to 15 minutes between rolling passes was able to be achieved without reducing the workability. The formation of the banded microstructure was slightly affected by the time reduction. In the samples rolled with 7 to 15 minutes internal anneal, the number and width of the clusters of large grains were increased, but they did not form lengthy bands.

All samples produced by all the conditions had an averaged grain size of about 10 μm. These finer grains were contributed by the smaller starting microstructures of the “deformed” dendrites.

The “band” microstructure can be detrimental to the ductility of the finished sheet along the rolling direction. The formation of this microstructure is related to the activation of a twinning deformation mechanism during the rolling process that introduces minor and major deformation zones that later recrystallize into alternate bands of large and fine grains, respectively, during the final anneal. Normally, twinning is the main mode of deformation in magnesium alloys when the deformation temperature is below about 320° C. The rolling mill therefore preferably has the capability to heat the rolls so that the temperature of the work piece will not drop below 320° C. during the rolling operation, at least if the pre-heating temperature and/or the roll speed are not sufficiently high to prevent the formation of the “banded” microstructure.

After the finish hot rolling, the resultant strip is subjected to a finish cold rolling stage. However, as detailed above, the finish hot rolling can be omitted, if required, for TRC strip. In each case, we have not found direct evidence to correlate the degree of grain refinement during recrystallization with the size and distribution of secondary particles in the TRC magnesium alloys. The principal parameter appears to be the amount and the distribution of stored deformation energy. Cold rolling is an effective method for providing high levels of such stored energy to induce recrystallization on subsequent heat treatment.

As detailed above, conventional processing of magnesium alloy in the finish treatment for producing sheet frequently uses a finish warm rolling stage. A finish cold rolling stage can be used, but necessitates only a low level of reduction per pass of 1% to 2%. However, in the process of the present invention, the finish cold rolling stage is not subject to such constraint. That stage in the present invention, with TRC strip which has either the equiaxed or deformed dendritic microstructure in its as cast condition, enables reduction levels of from 15% to 25% in each pass.

In trials with 120 mm wide and 0.7 to 0.75 mm thick sheet, produced by hot rolling at 420° C. from homogenized TRC strip which, as-cast, had an equiaxed dendritic microstructure, the sheet was heat treated at not more than 30 minutes at 420° C. and then cold rolled. During the cold rolling, the rolling mill was set such that there was no gap between the rolls, and the total reduction after three rolling passes was 15%. In other trials a total reduction of 25% was obtained after three cold rolling passes. In the latter case, the microstructure consisted of finer grains with a size down to about 3 μm and larger grains with a size up to 12 μm and an average grain size of 7 μm. In a further trial, a reduction of 20% was obtained in a single cold roll pass, to provide a microstructure with finer grains of less than 10 μm, and coarser grains up to 25 μm. The less uniform grain size after the single pass indicates that it is preferable to use multiple passes instead of a single pass to achieve a given total reduction.

It is indicated above that at hot rolling temperatures below 320° C., a banded microstructure can result. While this is undesirable, it is found that its effect is reduced by cross cold rolling to produce a regular “checkerboard” microstructure.

With samples similar to those detailed for cold rolling of sheet from equiaxed dendritic TRC strip, but with sheet of 0.7 mm to 0.75 mm obtained from deformed dendritic TRC strip, comparable results were obtained. Thus, with respective samples subjected to three cold rolling passes, a total reduction of 20% was obtained in one instance, while a reduction of 30% was obtained in the other. The increase in reduction from 20% to 30% was accompanied by a reduction in average grain size from 7 μm to 4 μm. However, there were more clusters of large grains in the samples reduced by 30%.

Further samples derived from TRC strip which, as-cast, had a deformed dendritic microstructure, exhibited bands of larger grains produced as a consequence of hot rolling at 350° C. These bands were found to persist after six cold rolling passes. However, it is found that cold rolling could eliminate most of the bands of large grains indicated above as being formed by a reduction in pre-hot rolling anneal time (such as from about 18 hours to 2 hours).

Still further samples derived from TRC strip of both deformed and equiaxed dendritic microstructures were subjected to rolling at room temperature with a degree of reduction between each pass at a constant level between 1% and 27%. These samples, as cast, were subjected to a homogenizing anneal at 350° C. or 420° C. for 12 to 18 hours and then to the cold rolling, without an intervening hot rolling stage. The samples were 200 mm long, 50 mm wide and 2.6 mm thick. At greater than 20% reduction per pass, a single pass was sufficient to introduce edge cracking. At a cold reduction of 14% per pass, two passes (for a total reduction of 24%) caused edge cracking. At a cold reduction of 10% to 13% per pass, three passes (for a total reduction of 35%) was able to be tolerated without edge cracking. At a cold reduction of 1% to 2% per pass, 30 passes could be conducted (for a total reduction of 46%) before edge cracks appeared. However, after reaching the maximum total reduction for any of these rolling sequences, annealing of the strip such as at 350° C. for 60 minutes or 420° C. for 30 minutes enabled cold rolling to recommence at similar rolling reductions without adverse effects.

The difference in the reduction per cold roll pass does not affect the final microstructure. For sheet produced with a thickness of 0.7 mm, and then annealed at 350° C. for 60 minutes, the microstructure can exhibit fine grains of 3 μm in size, clusters of larger grains of up to 10 μm and an average grain size of 5 μm.

Following the finish cold rolling, the as-rolled sheet is subjected to a finish anneal sufficient to achieve recrystallization. The duration of the anneal decreases with increase in temperature level, as indicated by the general suitability of for example 350° C. for less than about 60 minutes or 420° C. for less than about 30 minutes. Each of these treatments result in similar microstructures, although the latter treatment results in a larger grain size scatter. However ductility in the transverse direction is not adversely influenced by this difference.

In large part, the foregoing results have been established with trials conducted with AZ31B, AZ61, AZ91 and AM60 alloys. However, comparable results are indicated for magnesium alloys in general. For such alloys, the invention is expected to facilitate more simple, lower cost production of magnesium alloy sheet, with the process of the invention requiring equipment which has a substantially lower capital cost than is necessary in ingot based processing.

Finally, it is to be understood that various alterations, modifications and/or additions may be introduced into the constructions and arrangements of parts previously described without departing from the spirit or ambit of the invention. 

1. A method of producing magnesium alloy strip, suitable for use in the production of magnesium alloy sheet by rolling reduction and heat treatment, wherein the method includes the steps of: (a) casting magnesium alloy as strip, using a twin roll casting installation; and (b) controlling the thickness and temperature of the strip exiting from between rolls of the installation whereby the strip has a microstructure characterised by a primary phase having a form selected from deformed dendritic, equiaxed dendritic and a mixture of deformed and equiaxed dendritic forms.
 2. The method of claim 1, wherein the controlling of step (b) includes adjusting the spacing between the rolls to provide strip having a thickness of less than 10 mm.
 3. The method of claim 2, wherein the adjusting provides strip having a thickness of not more than about 7 mm.
 4. The method of claim 2, wherein the adjusting provides strip having a thickness of less than about 5 mm down to about 2.5 mm.
 5. The method of claim 1, wherein the controlling of step (b) results in the strip exiting from between the rolls at a temperature of about 200° C. to about 350° C.
 6. The method of claim 1, wherein the controlling of step (b) results in the strip exiting from between the rolls at a temperature of about 200° C. to about 260° C.
 7. The method of claim 1, wherein the controlling of step (b) is conducted to provide strip exiting from between the rolls at a temperature of from about 200° C. to 220° C. and a thickness of from about 4 mm to about 5 mm, whereby the strip has a microstructure characterised by deformed dendritic primary phase substantially free of equiaxed dendritic primary phase.
 8. The method of claim 1, wherein the controlling of step (b) is conducted to provide strip exiting from between the rolls at a temperature of from about 200° C. to 245° C. and a thickness less than about 4 mm, whereby the strip has a microstructure characterised by deformed dendritic primary phase substantially free of equiaxed dendritic primary phase.
 9. The method of claim 1, wherein the controlling of step (b) is conducted to provide strip exiting from between the rolls at a temperature of at least 230° C. and a thickness of from about 4 mm to about 5 mm, whereby the strip has a microstructure substantially characterised by equiaxed dendritic primary phase.
 10. The method of claim 9, wherein the temperature is from about 230° C. to about 240° C.
 11. The method of claim 1, wherein the controlling of step (b) is conducted to provide strip exiting from between the rolls at a temperature of at least 245° C. and with a thickness of less than about 4 mm, whereby the strip has a microstructure substantially characterised by equiaxed dendritic primary phase.
 12. The method of claim 9, wherein the equiaxed dendritic primary phase is characterised by grains which are of spheroidal form.
 13. The method of claim 7, wherein the deformed dendritic primary phase is characterised by grains which, while reflecting dendritic growth, are of an elongate flattened form extending in the rolling direction, substantially parallel to major surfaces of the strip.
 14. The method of claim 1, wherein the strip is subjected to a homogenizing heat treatment to achieve recrystallization to a required grain size.
 15. The method of claim 14, wherein the homogenizing heat treatment is conducted at a temperature of from about 330° C. to 500° C.
 16. The method of claim 14, wherein the homogenizing heat treatment is conducted at a temperature of from about 400° C. to 500° C.
 17. The method of claim 14, wherein the strip passes from the twin roll casting installation to a furnace in which the homogenizing heat treatment is conducted, whereby loss of heat energy from the strip prior to the heat treatment is minimised.
 18. The method of claim 14, wherein the strip is held at an intermediate temperature, which is suitable for reducing segregation by secondary phase being taken into solid solution, before the strip is heated to the homogenizing temperature.
 19. The method of claim 18, wherein the intermediate temperature is from about 340° C. to 360° C.
 20. A method of producing magnesium alloy sheet, wherein the method includes the steps of: (a) casting magnesium alloy as strip, using a twin roll casting installation; (b) controlling the thickness and temperature of the strip exiting from between rolls of the installation whereby the strip has a microstructure characterised by a primary phase having a form selected from deformed dendritic, equiaxed dendritic and a mixture of deformed and equiaxed dendritic forms; (c) subjecting the strip to a homogenizing heat treatment to achieve full or partial recrystallization of the microstructure to a required grain size; (d) rolling the homogenized strip to produce magnesium alloy sheet of a required gauge; and (e) annealing the sheet produced by step (d).
 21. The method of producing magnesium alloy sheet, wherein the method includes the steps of: (i) producing magnesium alloy strip by the method of claim 2; (ii) subjecting the strip to a homogenizing heat treatment to achieve full or partial recrystallization of the microstructure to a required grain size; (iii) rolling the homogenized strip to produce magnesium alloy sheet of a required gauge; and (iv) annealing the sheet produced by step (iii).
 22. The method of producing magnesium alloy sheet, wherein the method includes the steps of: (i) producing magnesium alloy strip by the method of claim 15; (ii) subjecting the strip to a homogenizing heat treatment to achieve full or partial recrystallization of the microstructure to a required grain size; (iii) rolling the homogenized strip to produce magnesium alloy sheet of a required gauge; and (iv) annealing the sheet produced by step (iii).
 23. The method of claim 20, wherein the homogenizing heat treatment is conducted at a temperature and for a period sufficient to substantially eliminate segregation of deformed dendritic primary phase.
 24. The method of claim 23, wherein the strip on completion of the homogenizing heat treatment is characterized by a microstructure consisting substantially of fine grains of from about 10 μm to about 15 μm in size resulting from recrystallization of deformed dendritic microstructure.
 25. The method of claim 20, wherein the strip on completion of the homogenizing heat treatment is characterized by a microstructure consisting of grains of from 50 μm to about 200 μm in size resulting from recrystallization of deformed dendritic microstructure.
 26. The method of claim 20, wherein the step of rolling the homogenized strip includes stages of finish cold rolling, followed by an anneal heat treatment.
 27. The method of claim 26, wherein the step of rolling includes, before the finish cold rolling, a stage of finish hot rolling.
 28. The method of claim 27, wherein the finish hot rolling is conducted at a temperature at which hot rolling causes recrystallization of the microstructure.
 29. The method of claim 28, wherein the finish hot rolling is conducted at a temperature of from 200° C. to 500° C.
 30. The method of claim 28, wherein the finish hot rolling is conducted at a temperature of from 350° C. to 500° C.
 31. The method of claim 28, wherein the finish hot rolling is conducted at a temperature of from 400° C. to 500° C.
 32. The method of claim 27, wherein the finish hot rolling achieves from 20% to 25% thickness reduction per pass of hot rolling.
 33. The method of claim 26, wherein the finish cold rolling achieves from 15% to 25% thickness reduction per pass of cold rolling.
 34. The method of claim 26, wherein the anneal heat treatment involves an inverse temperature/time relationship of the order of about 350° C. for less than about 60 minutes or 420° C. for less than about 30 minutes, suitable for achieving recrystallization of the microstructure.
 35. Magnesium alloy strip produced by the method of claim
 1. 36. Magnesium alloy sheet produced by the method of claim
 20. 